SULPHUR hexafluoride is widely used as an insulator in many electrical devices, but little is known of its fundamental electrical properties, particularly at voltages above about 100 kV. As the first part of an investigation on the growth of pre-breakdown ionization in SF6, we have carried out measurements from which values of the apparent primary ionization coefficient α′ have been determined, at pressures in the range from 50 to 400 torr and for voltages up to about 200 kV. We report here a dependence of the coefficient α′ on gas pressure, not previously observed in SF6, which is significant to the insulating properties of the gas and which can be related to the fundamental collision processes occurring.